The mechanical seal described by the present applicant in Domestic Republication of PCT International Application No. 2009-008289 (Patent Document 1) (hereinafter called “the prior art”) has been known in the past as a mechanical seal suitable for sealing in slurries or other fluids.
FIG. 6 is a vertical cross-sectional view illustrating the prior art.
A mechanical seal device 50 comprises a machine-interior mechanical seal 60 and a machine-exterior (atmosphere-side) mechanical seal 70.
The machine-exterior mechanical seal 60 comprises a rotary-side sealing ring 61 arranged on the machine-interior side and a stationary-side sealing ring 62 arranged on the machine-exterior side. In the machine-interior mechanical seal 60, the stationary-side sealing ring 62 is pushed in the direction of the rotary-side sealing ring 61 by a spring 63 mounted in a first seal case 65 and by a packing 64 made of a rubber material, and a sealing sliding surface is formed.
The machine-exterior mechanical seal 70 comprises a rotary-side sealing ring 71 arranged on the machine interior side and a stationary-side sealing ring 72 arranged on the machine exterior side. In the machine-exterior mechanical seal 70 as well, the stationary-side sealing ring 72 is pushed in the direction of the rotary-side sealing ring 71 by a spring 73 mounted on a second seal case 74, and a sealing sliding surface is formed.
In the machine-interior mechanical seal 60, a ring-shaped projection 66 projecting toward the back surface of the packing 64 is provided to the first seal case 65. A configuration is adopted such that, when the apparatus is assembled, a predetermined gap α will be present between the ring-shaped projection 66 and the back surface of the ring-shaped packing 64. During operation, when the pressure on the machine-interior increases, an inner circumferential side of the ring-shaped packing 64 deforms toward the machine-exterior side, but at that time the ring-shaped projection 66 provided to the first seal case 65 supports the back surface of the ring-shaped packing 64, and serves to hinder any deformation of the packing 64 beyond the gap α.
When the sealed fluid is a slurry, the slurry may stick to the spring 63 mounted on the first seal case 65 of the machine-interior mechanical seal 60, and thereafter cling thereto. In such instances, the spring 63 is entirely unable to function, and the ring-shaped rubber packing 64 alone is solely responsible for the function of pressing the stationary-side sealing ring 62 toward the sliding surface.
FIG. 7 is a descriptive drawing illustrating the stationary-side sealing ring 62 and the packing 64 in a zero-load state, a low-load state, and a high-load (high-pressure) state in a case in the prior art where the spring 63 is entirely unable to function.
FIG. 7(a) illustrates the no-load state and the low-load state; in these states, the stationary-side sealing ring 62 is pushed in the direction of the rotary-side sealing ring 61 by the ring-shaped packing 64, and the sealing sliding surface is formed.
FIGS. 7(b) and 7(c) are illustrations of the high-load (high-pressure) state, where FIG. 7(b) illustrates a case in which the inner circumference of the packing 64 has been coated with grease and FIG. 7(c) illustrates a case in which the inner circumference of the packing 64 has not been coated with grease.
In the case in FIG. 7(b) where the inner circumference of the packing 64 has been coated with grease, the coefficient of friction between the inner circumference of the packing 64 and the stationary-side sealing ring 62 is lowered and the pressure of the machine-interior side acts on the machine-interior side of the packing 64; the packing 64 deforms, and the inner circumferential side, which is a free end thereof, attempts to withdraw. At this time, because the coefficient of friction between the inner circumference of the packing 64 and the stationary-side sealing ring 62 is low, only the inner circumferential side of the packing 64 withdraws, sliding with respect to the stationary-side sealing ring 62, but the sliding surface between the stationary-side sealing ring 62 and the rotary-side sealing ring 61 just manages to achieve a state where a seal is formed, due to the pressure of the sealed fluid.
By contrast, in the case in FIG. 7(c) where the inner circumference of the packing 64 is not coated with grease, the coefficient of friction between the inner circumference of the packing 64 and the stationary-side sealing ring 62 is high, and when a certain pressure is reached, the pressure whereby the inner circumferential side of the packing 64 withdraws becomes greater than the pressure pressing on the stationary-side sealing ring 62 toward the sliding surface. The stationary-side sealing ring 62 withdraws together with the inner circumferential side of the packing 64, and the sliding surface between the stationary-side sealing ring 62 and the rotary-side sealing ring 61 opens and enters a state where leakage occurs.